The present invention is directed to an electrical field device of the pressure-resistant encapsulation type of protection for use in areas at risk of explosion.
Many field devices are used in areas at risk of explosion wherein, for example, combustible liquids or gasses are stored, transported and/or processed, so that the risk of an explosive atmosphere is present Special protective measures are therefore required in order to avoid a possible explosion. Two fundamentally different concepts that require both different field devices and electrical signals as well as a different installation of the lines are thereby essentially applied in the installation of field devices in such areas at risk of explosion.
Field devices according to a first concept, namely that of the self-protected type of protection having the European code system or designation and "EEx i" are mainly employed in Europe. This type of device measures, such as, for example, casting out or removing electronic components of the protective circuits of, typically, diodes and resistors assured that no energy that could trigger an explosion can arise locally even in case of damage. The energy transported on the signal lines is therefore likewise limited. The separation of the circuit into a secure area and an area at risk of explosion is realized with safety barriers or feed separators that are arranged in the secure area.
The use of field devices according to a second concept is also standard, namely that of the pressure-resistant encapsulation having the European code system "EEx d", which is widespread particularly in the USA, in Arabia and in Italy as well. A self-protected field device is thereby constructed by design measures of essentially the housing so that a spark or, respectively, a flame within the housing would also not be in the position to trigger damage outside the housing. Field devices of this second type of protection therefore usually comprise solid housings whose joints are characterized by screw threads or long, narrow gaps, and comprise displays that are protected by correspondingly thick windows of, for example, bullet-proof glass. Housings for the type of protection of pressure-resistant encapsulation "EEx d" are thus generally heavy as well as expensive and dare not be opened during operation. The installation of lines in the area at risk of explosion is thereby often provided in pipelines, whereby the pipes are then screwed to the field devices. A general limitation of the energy on the lines or a separation of the circuits into a secure area and an area at risk of explosion, however, is not required.
It is generally not desirable to utilize field devices according to the two concepts described above, namely field devices of the self-secured type of protection on the one hand and field devices of the type of protection of the pressure-resistant encapsulation on the other hand, with an area of an industrial system at .sub.1 risk of explosion since these two different field devices, in particular, require a different installation of the lines, control boxes or, respectively, terminal housing and an operator is usually only familiar with one type of installation. Deriving therefrom for manufacturers of field devices is the difficult situation that essentially two field device types differing completely in terms of electronics and housing must be developed and manufactures, namely one per type of protection. Disadvantageously, many user-friendly properties with possibilities of paramertization and control--as known from self-secured field devices--are thereby often not realized in the design of field devices of the type of protection of the pressure-resistant encapsulation "EEx d" since operating and adjustment devices lying within the housing are generally not accessible.
Further, housings for meeting the explosion-protection demands "EEx d", i.e. of the type of protection of the pressure-resistant encapsulation "Ex d", are known for the installation of commercially available controls or electrical devices such as, for example, switches, even with the possibility for flanged attachment.